The present invention relates to a process for producing a titanium dioxide pigment useful for photograph and a photographic support comprising it. More particularly, it relates to a photographic support in production of which occurrence of die lip stain and microgrit is substantially retained, and which meets requirements of a higher sharpness.
Since resin-coated paper type photographic supports were developed several ten years ago, various patent applications have been made thereon and considerable improvements have been made in their quality and in production technique thereof.
It is not too much to say that all of the problems relatively easy to improve have been solved until now.
At present, important problems which have not yet been fully improved in spite of the efforts made by the manufacturers in this field are so-called die lip stain and microgrit in production thereof.
Patent applications on solution of the problem of die lip stain (die lip streaks) were filed in comparatively earlier time. For example, a method in which zinc oxide and a specific dispersant are contained in a composition of a resin and a titanium dioxide pigment is proposed in Japanese Patent Application Kokai No. 53-02947. For improvement in respect of resin, Japanese Patent Application Kokai No. 57-109948 discloses to limit a specific characteristic value n and for improvement in respect of a titanium dioxide pigment, Japanese Patent Application Kokai No. 58-17433 discloses to use titanium dioxide pigments in which the surface of particles is coated with di- tetra-hydric alcohols.
However, solution of the problem of die lip stain is so difficult that satisfactory photographic support has not yet been obtained by only improving materials and use of materials in combination.
The microgrit which originates from resin per se has been the problem called resin gel in the field of resins for a long time. Furthermore, in the case of masterbatch containing a titanium dioxide pigment, it has been known as microgrit. This problem is described, for example, in Japanese Patent Application Kokai No. 59-1544, pages 3-4.
Although not concerning with a process for producing a titanium dioxide pigment for photograph, Japanese Patent Application Kokoku No. 50-36440 discloses a process for producing a pigment titanium dioxide, which comprises the steps of
(a) hydrolyzing a solution of titanium sulfate to produce a precipitate of hydrated titanium dioxide, PA1 (b) washing the precipitate, PA1 (c) treating the thus washed precipitate with at least one treating agent selected from lithium, beryllium, aluminum, magnesium, zinc, sodium, potassium, rubidium, cesium, and compounds of these metals, PA1 (d) treating the precipitate with a volatile ammonium compound, and PA1 (e) introducing the precipitates obtained in the steps (c) and (d) into futile forming seed and calcining it at 750.degree.-1000.degree. C., PA1 wherein pH of the above wet precipitate is 6-10 just before the calcination and the precipitate just before the calcination is accompanied by PA1 (a) a step of adding at least one alkaline earth metal compound in an amount of 0.01-2.0% by weight in terms of a metal oxide based on the titanium dioxide before calcination step and PA1 (b) an acid-washing step of washing the titanium dioxide at acidic state after the calcination step and before a step of surface treatment with a hydrated metal oxide, PA1 and wherein particle size of the titanium dioxide pigment is 0.110-0.150 .mu.m, shown by number-average diameter obtained by measuring the diameter in a given direction using an electron microscope. PA1 Raw titanium dioxide.fwdarw.digestion, extraction.fwdarw.allowing to stand, crystallization.fwdarw.filtration, concentration.fwdarw.hydrolyzation.fwdarw.washing.fwdarw.calcination.fwdar w.grinding, screening.fwdarw.wet grinding, classification.fwdarw.surface treatment.fwdarw.washing, drying.fwdarw.finishing grinding.fwdarw.titanium dioxide pigment
(i) at least 0.7% by weight in total of volatile ammonium compound (one or more kinds) (in terms of ammonium sulfate based on the weight of titanium dioxide), PA2 (ii) totally 0.02-0.50% by weight of a treating agent selected from beryllium, aluminum, magnesium, zinc, and compounds of these metals (in terms of metal oxide based on the weight of titanium dioxide), and PA2 (iii) 0.05-1.0% by weight of a treating agent selected from the group consisting of sodium, potassium, rubidium, cesium, and compounds of these alkali metals (in terms of alkali metal oxide based on the weight of titanium dioxide), with a proviso that lithium and/or at least one lithium compound may be substituted with one or all of the treating agents selected from the group referred to in the above (ii) and/or (iii).
This patent publication discloses treatment with lithium and other several kinds of metals and compounds thereof, but also discloses that lithium and/or at least one lithium compound is partially or totally substituted in preference to the other metals and compounds thereof. The publication makes no mention of limitation to a specific particle size and besides, the objects thereof are improvements of lightness (L value), coloring power and texture.
Furthermore, Russian Patent No. 975576 discloses a rutile type titanium dioxide pigment prepared through an acid washing step after a calcination step as a titanium dioxide pigment which can be used for making a photographic paper. However, this patent makes mention of neither die lip stain nor microgrit. Further, the object thereof is to reduce photographic activity. This is referred to on pages 14-16 of Japanese Patent Application No. H.1-58943.
Taking into consideration the above prior art, problems (tasks) to be solved by the present invention will be explained in detail.
Recently, demand for photographic supports of high quality, for example, of high sharpness has more and more increased and tendency to increase content of a titanium dioxide pigment in a resin layer of a photographic support has increased.
When concentration of a titanium dioxide pigment in a resin layer of a thermoplastic resin-coated paper is raised, a photographic support on which a printed image high in sharpness can be obtained, but serious problems often occur in photographic properties and production technique. That is, when a thermoplastic resin composition, especially a polyolefin resin composition, which contains a titanium dioxide pigment is melt extruded into a film from a slit die and applied to a substrate, a sticking matter or a stain in the form of a needle or an icile (hereinafter referred to as "die lip stain") tends to be formed on the tip of die lip after a short run extrusion. Since the die lip stain tends to grow larger with lapse of time of the run, it is very troublesome.
If this die lip stain occurs during melt extrusion coating, streaks are generated on the surface of the thus produced thermoplastic resin-coated paper or the stain sometimes comes off and sticks to the resin layer, resulting in a foreign matter. Therefore, the surface property of the thermoplastic resin-coated paper is seriously damaged, and such paper is utterly unsuitable as a photographic support which requires excellent surface property and has no commercial value. In order to remove completely the die lip stain once formed, the production has to be shut down to clean the die lip or exchange the die. The cleaning or exchanging requires much labor and time, resulting in considerable reduction of productivity.
A further problem is that in a photographic support coated with a thermoplastic resin composition, especially a polyolefin resin composition, which contains a titanium dioxide pigment, fine foreign matters or particles called microgrit are apt to be formed on the surface of the resin containing the titanium dioxide pigment on the image forming side of the support.
If microgrit is formed on a resin-coated paper used as a photographic support, serious photographic problems are brought about. That is, when a portrait is developed on a photographic paper using a photographic support of a resin-coated paper having microgrit, and if the microgrit appears in the portion of a face or the like, commercial value of the paper is completely lost. This microgrit also tends to be generated more often when concentration of a titanium dioxide pigment in the resin layer of the thermoplastic resin-coated paper is increased.